Method and system for forming high accuracy patterns using charged particle beam lithography

ABSTRACT

A method and system for fracturing or mask data preparation for charged particle beam lithography are disclosed in which a plurality of charged particle beam shots is determined that will form a pattern on a surface using a multi-beam charged particle beam writer, where the sensitivity of the pattern on the surface to manufacturing variation is reduced by increasing edge slope.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/106,584 filed on Dec. 13, 2013 entitled “Method and SystemFor Forming High Accuracy Patterns Using Charged Particle BeamLithography”, which is hereby incorporated by reference for allpurposes. U.S. patent application Ser. No. 14/106,584 1) is acontinuation of U.S. patent application Ser. No. 13/924,019 filed onJun. 21, 2013 entitled “Method and System For Forming Patterns UsingCharged Particle Beam Lithography With Multiple Exposure Passes” andissued as U.S. Pat. No. 8,612,901; which 2) is a continuation of U.S.patent application Ser. No. 13/168,954 entitled “Method and System ForForming High Accuracy Patterns Using Charged Particle Beam Lithography”filed on Jun. 25, 2011 and issued as U.S. Pat. No. 8,473,875; which 3)claims priority to U.S. Provisional Patent Application Ser. No.61/392,477 filed on Oct. 13, 2010 and entitled “Method for IntegratedCircuit Manufacturing and Mask Data Preparation Using CurvilinearPatterns”; and 4) is related to U.S. patent application Ser. No.13/168,953 filed on Jun. 25, 2011 and issued as U.S. Pat. No. 8,703,389,entitled “Method and System for Forming Patterns with Charged ParticleBeam Lithography”; all of which are hereby incorporated by reference forall purposes.

U.S. patent application Ser. No. 14/106,584: 5) is also acontinuation-in-part of U.S. patent application Ser. No. 13/723,181filed on Dec. 20, 2012 entitled “Method For Forming Circular Patterns OnA Surface” and issued as U.S. Pat. No. 8,609,306; which 6) is acontinuation of U.S. patent application Ser. No. 13/282,446 filed onOct. 26, 2011 entitled “Method, Device, And System For Forming CircularPatterns On A Surface” and issued as U.S. Pat. No. 8,354,207; which 7)is a continuation of U.S. patent application Ser. No. 12/540,322 filedon Aug. 12, 2009 entitled “Method and System For Forming CircularPatterns On a Surface” and issued as U.S. Pat. No. 8,057,970, all ofwhich are hereby incorporated by reference for all purposes.

U.S. patent application Ser. No. 12/540,322: 8) is acontinuation-in-part of U.S. patent application Ser. No. 12/202,364filed Sep. 1, 2008, entitled “Method and System For Manufacturing aReticle Using Character Projection Particle Beam Lithography” and issuedas U.S. Pat. No. 7,759,026; 9) is a continuation-in-part of U.S. patentapplication Ser. No. 12/473,241 filed May 27, 2009, entitled “Method forManufacturing a Surface and Integrated Circuit Using Variable ShapedBeam Lithography” and issued as U.S. Pat. No. 7,754,401; 10) claimspriority from U.S. Provisional Patent Application Ser. No. 61/224,849filed Jul. 10, 2009, entitled “Method and System for ManufacturingCircular Patterns On a Surface And Integrated Circuit”; and 11) isrelated to U.S. patent application Ser. No. 12/540,321 filed Aug. 12,2009, entitled “Method For Fracturing Circular Patterns and ForManufacturing a Semiconductor Device” and issued as U.S. Pat. No.8,017,288; all of which are hereby incorporated by reference for allpurposes.

BACKGROUND OF THE DISCLOSURE

The present disclosure is related to lithography, and more particularlyto the design and manufacture of a surface which may be a reticle, awafer, or any other surface, using charged particle beam lithography.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, optical lithography may be used to fabricate thesemiconductor devices. Optical lithography is a printing process inwhich a lithographic mask or photomask manufactured from a reticle isused to transfer patterns to a substrate such as a semiconductor orsilicon wafer to create the integrated circuit (I.C.). Other substratescould include flat panel displays, holographic masks, or even otherreticles. While conventional optical lithography uses a light sourcehaving a wavelength of 193 nm, extreme ultraviolet (EUV) or X-raylithography are also considered types of optical lithography in thisapplication. The reticle or multiple reticles may contain a circuitpattern corresponding to an individual layer of the integrated circuit,and this pattern can be imaged onto a certain area on the substrate thathas been coated with a layer of radiation-sensitive material known asphotoresist or resist. Once the patterned layer is transferred the layermay undergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Eventually, a combination of multiplesof devices or integrated circuits will be present on the substrate.These integrated circuits may then be separated from one another bydicing or sawing and then may be mounted into individual packages. Inthe more general case, the patterns on the substrate may be used todefine artifacts such as display pixels, holograms, directedself-assembly (DSA) guard bands, or magnetic recording heads.Conventional optical lithography writing machines typically reduce thephotomask pattern by a factor of four during the optical lithographicprocess. Therefore, patterns formed on the reticle or mask must be fourtimes larger than the size of the desired pattern on the substrate orwafer.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, non-optical methods may be used to transfer apattern on a lithographic mask to a substrate such as a silicon wafer.Nanoimprint lithography (NIL) is an example of a non-optical lithographyprocess. In nanoimprint lithography, a lithographic mask pattern istransferred to a surface through contact of the lithography mask withthe surface.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, maskless direct write may also be used to fabricatethe semiconductor devices. Maskless direct write is a printing processin which charged particle beam lithography is used to transfer patternsto a substrate such as a semiconductor or silicon wafer to create theintegrated circuit. Other substrates could include flat panel displays,imprint masks for nano-imprinting, or even reticles. Desired patterns ofa layer are written directly on the surface, which in this case is alsothe substrate. Once the patterned layer is transferred the layer mayundergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Some of the layers may be written usingoptical lithography while others may be written using maskless directwrite to fabricate the same substrate. Also, some patterns of a givenlayer may be written using optical lithography, and other patternswritten using maskless direct write. Eventually, a combination ofmultiples of devices or integrated circuits will be present on thesubstrate. These integrated circuits are then separated from one anotherby dicing or sawing and then mounted into individual packages. In themore general case, the patterns on the surface may be used to defineartifacts such as display pixels, holograms, or magnetic recordingheads.

Two common types of charged particle beam lithography are variableshaped beam (VSB) and character projection (CP). These are bothsub-categories of shaped beam charged particle beam lithography, inwhich a precise electron beam is shaped and steered so as to expose aresist-coated surface, such as the surface of a wafer or the surface ofa reticle. In VSB, these shapes are simple shapes, usually limited torectangles of certain minimum and maximum sizes and with sides which areparallel to the axes of a Cartesian coordinate plane (i.e. of“Manhattan” orientation), and 45 degree right triangles (i.e. triangleswith their three internal angles being 45 degrees, 45 degrees, and 90degrees) of certain minimum and maximum sizes. At predeterminedlocations, doses of electrons are shot into the resist with these simpleshapes. The total writing time for this type of system increases withthe number of shots. In character projection (CP), there is a stencil inthe system that has in it a variety of apertures or characters which maybe complex shapes such as rectilinear, arbitrary-angled linear,circular, nearly circular, annular, nearly annular, oval, nearly oval,partially circular, partially nearly circular, partially annular,partially nearly annular, partially nearly oval, or arbitrarycurvilinear shapes, and which may be a connected set of complex shapesor a group of disjointed sets of a connected set of complex shapes. Anelectron beam can be shot through a character on the stencil toefficiently produce more complex patterns on the reticle. In theory,such a system can be faster than a VSB system because it can shoot morecomplex shapes with each time-consuming shot. Thus, an E-shaped patternshot with a VSB system takes four shots, but the same E-shaped patterncan be shot with one shot with a character projection system. Note thatVSB systems can be thought of as a special (simple) case of characterprojection, where the characters are just simple characters, usuallyrectangles or 45-45-90 degree triangles. It is also possible topartially expose a character. This can be done by, for instance,blocking part of the particle beam. For example, the E-shaped patterndescribed above can be partially exposed as an F-shaped pattern or anI-shaped pattern, where different parts of the beam are cut off by anaperture. This is the same mechanism as how various sized rectangles canbe shot using VSB. In this disclosure, partial projection is used tomean both character projection and VSB projection. Shaped beam chargedparticle beam lithography may use either a single shaped beam, or mayuse a plurality of shaped beams simultaneously exposing the surface, theplurality of shaped beams producing a higher writing speed than a singleshaped beam.

As indicated, in lithography the lithographic mask or reticle comprisesgeometric patterns corresponding to the circuit components to beintegrated onto a substrate. The patterns used to manufacture thereticle may be generated utilizing computer-aided design (CAD) softwareor programs. In designing the patterns the CAD program may follow a setof pre-determined design rules in order to create the reticle. Theserules are set by processing, design, and end-use limitations. An exampleof an end-use limitation is defining the geometry of a transistor in away in which it cannot sufficiently operate at the required supplyvoltage. In particular, design rules can define the space tolerancebetween circuit devices or interconnect lines. The design rules are, forexample, used to ensure that the circuit devices or lines do notinteract with one another in an undesirable manner. For example, thedesign rules are used so that lines do not get too close to each otherin a way that may cause a short circuit. The design rule limitationsreflect, among other things, the smallest dimensions that can bereliably fabricated. When referring to these small dimensions, oneusually introduces the concept of a critical dimension. These are, forinstance, defined as the smallest width of a line or the smallest spacebetween two lines, those dimensions requiring exquisite control.

One goal in integrated circuit fabrication by optical lithography is toreproduce the original circuit design on the substrate by use of thereticle. Integrated circuit fabricators are always attempting to use thesemiconductor wafer real estate as efficiently as possible. Engineerskeep shrinking the size of the circuits to allow the integrated circuitsto contain more circuit elements and to use less power. As the size ofan integrated circuit critical dimension is reduced and its circuitdensity increases, the critical dimension of the circuit pattern orphysical design approaches the resolution limit of the optical exposuretool used in conventional optical lithography. As the criticaldimensions of the circuit pattern become smaller and approach theresolution value of the exposure tool, the accurate transcription of thephysical design to the actual circuit pattern developed on the resistlayer becomes difficult. To further the use of optical lithography totransfer patterns having features that are smaller than the lightwavelength used in the optical lithography process, a process known asoptical proximity correction (OPC) has been developed. OPC alters thephysical design to compensate for distortions caused by effects such asoptical diffraction and the optical interaction of features withproximate features. OPC includes all resolution enhancement technologiesperformed with a reticle.

OPC may add sub-resolution lithographic features to mask patterns toreduce differences between the original physical design pattern, thatis, the design, and the final transferred circuit pattern on thesubstrate. The sub-resolution lithographic features interact with theoriginal patterns in the physical design and with each other andcompensate for proximity effects to improve the final transferredcircuit pattern. One feature that is used to improve the transfer of thepattern is a sub-resolution assist feature (SRAF). Another feature thatis added to improve pattern transference is referred to as “serifs”.Serifs are small features that can be positioned on an interior orexterior corner of a pattern to sharpen the corner in the finaltransferred image. It is often the case that the precision demanded ofthe surface manufacturing process for SRAFs is less than the precisiondemanded for patterns that are intended to print on the substrate, oftenreferred to as main features. Serifs are a part of a main feature. Asthe limits of optical lithography are being extended far into thesub-wavelength regime, the OPC features must be made more and morecomplex in order to compensate for even more subtle interactions andeffects. As imaging systems are pushed closer to their limits, theability to produce reticles with sufficiently fine OPC features becomescritical. Although adding serifs or other OPC features to a mask patternis advantageous, it also substantially increases the total feature countin the mask pattern. For example, adding a serif to each of the cornersof a square using conventional techniques adds eight more rectangles toa mask or reticle pattern. Adding OPC features is a very laborious task,requires costly computation time, and results in more expensivereticles. Not only are OPC patterns complex, but since optical proximityeffects are long range compared to minimum line and space dimensions,the correct OPC patterns in a given location depend significantly onwhat other geometry is in the neighborhood. Thus, for instance, a lineend will have different size serifs depending on what is near it on thereticle. This is even though the objective might be to produce exactlythe same shape on the wafer. These slight but critical variations areimportant and have prevented others from being able to form reticlepatterns. It is conventional to discuss the OPC-decorated patterns to bewritten on a reticle in terms of main features, that is features thatreflect the design before OPC decoration, and OPC features, where OPCfeatures might include serifs, jogs, and SRAF. To quantify what is meantby slight variations, a typical slight variation in OPC decoration fromneighborhood to neighborhood might be 5% to 80% of a main feature size.Note that for clarity, variations in the design of the OPC are what isbeing referenced. Manufacturing variations such as corner rounding willalso be present in the actual surface patterns. When these OPCvariations produce substantially the same patterns on the wafer, what ismeant is that the geometry on the wafer is targeted to be the samewithin a specified error, which depends on the details of the functionthat that geometry is designed to perform, e.g., a transistor or a wire.Nevertheless, typical specifications are in the 2%-50% of a main featurerange. There are numerous manufacturing factors that also causevariations, but the OPC component of that overall error is often in therange listed. OPC shapes such as sub-resolution assist features aresubject to various design rules, such as a rule based on the size of thesmallest feature that can be transferred to the wafer using opticallithography. Other design rules may come from the mask manufacturingprocess or, if a character projection charged particle beam writingsystem is used to form the pattern on a reticle, from the stencilmanufacturing process. It should also be noted that the accuracyrequirement of the SRAF features on the mask may be lower than theaccuracy requirements for the main features on the mask. As processnodes continue to shrink, the size of the smallest SRAFs on a photomaskalso shrinks. For example, at the 20 nm logic process node, 40 nm to 60nm SRAFs are needed on the mask for the highest precision layers.

Inverse lithography technology (ILT) is one type of OPC technique. ILTis a process in which a pattern to be formed on a reticle is directlycomputed from a pattern which is desired to be formed on a substratesuch as a silicon wafer. This may include simulating the opticallithography process in the reverse direction, using the desired patternon the substrate as input. ILT-computed reticle patterns may be purelycurvilinear—i.e. completely non-rectilinear—and may include circular,nearly circular, annular, nearly annular, oval and/or nearly ovalpatterns. Since these ideal ILT curvilinear patterns are difficult andexpensive to form on a reticle using conventional techniques,rectilinear approximations or rectilinearizations of the curvilinearpatterns may be used. The rectilinear approximations decrease accuracy,however, compared to the ideal ILT curvilinear patterns. Additionally,if the rectilinear approximations are produced from the ideal ILTcurvilinear patterns, the overall calculation time is increased comparedto ideal ILT curvilinear patterns. In this disclosure ILT, OPC, sourcemask optimization (SMO), and computational lithography are terms thatare used interchangeably.

EUV optical lithography has a much higher resolution than conventionaloptical lithography. The very high resolution of EUV significantlyreduces the need for OPC processing, resulting in lower mask complexityfor EUV than for 193 nm optical lithography. However, because of thevery high resolution of EUV, imperfections in a photomask, such asexcessive line edge roughness (LER), will be transferred to the wafer.Therefore, the accuracy requirements for EUV masks are higher than thosefor conventional optical lithography. Additionally, even though EUV maskshapes are not complicated by the addition of complex SRAFs or serifsrequired for conventional 193 nm lithography, EUV mask shapes arecomplicated by an addition of some complexities unique to EUVmanufacturing. Of particular relevance in writing patterns on masks forEUV lithography is mid-range scattering of charged particles such aselectrons, which may affect a radius of about 2 um. This mid-rangescattering introduces a new consideration for mask data preparation,because for the first time the influence from neighboring patterns hassignificant impact on the shape that a particular pattern would castonto the mask surface. Previously, when exposing masks for use withconventional 193 nm lithography, the short-range scattering affectedonly the pattern being written, and the long-range scattering had alarge enough effective range that only the size of a pattern, and notits detailed shape, was affected, making it possible to make correctionsby only using dose modulation. In addition, since EUV processing ofwafers is more expensive, it is desirable to reduce or eliminatemultiple patterning. Multiple patterning is used in conventional opticallithography to allow exposure of small features by exposing patterns forone layer of wafer processing using multiple masks, each of whichcontains a portion of the layer pattern. Reducing or eliminatingmultiple exposures requires the single mask to contain more finepatterns. For example, a series of collinear line segments may bedouble-patterned by first drawing a long line, then cutting the lineinto line segments by a second mask in conventional lithography. Thesame layer written with a single mask, such as for EUV lithography,would require a mask containing many smaller line segments. The need towrite larger numbers of finer patterns on a single mask, each patternneeding to be more accurate, increases the need for precision on EUVmasks.

There are a number of technologies used for forming patterns on areticle, including using optical lithography or charged particle beamlithography. The most commonly used system is the variable shaped beam(VSB), where, as described above, doses of electrons with simple shapessuch as Manhattan rectangles and 45-degree right triangles expose aresist-coated reticle surface. In conventional mask writing, the dosesor shots of electrons are designed to avoid overlap wherever possible,so as to greatly simplify calculation of how the resist on the reticlewill register the pattern. Similarly, the set of shots is designed so asto completely cover the pattern area that is to be formed on thereticle. U.S. Pat. No. 7,754,401 discloses a method of mask writing inwhich intentional shot overlap for writing patterns is used. Whenoverlapping shots are used, charged particle beam simulation can be usedto determine the pattern that the resist on the reticle will register.Use of overlapping shots may allow patterns to be written with reducedshot count or higher accuracy or both. U.S. Pat. No. 7,754,401 alsodiscloses use of dose modulation, where the assigned dosages of shotsvary with respect to the dosages of other shots. The term model-basedfracturing is used to describe the process of determining shots usingthe techniques of U.S. Pat. No. 7,754,401.

Reticle writing for the most advanced technology nodes typicallyinvolves multiple passes of charged particle beam writing, a processcalled multi-pass exposure, whereby the given shape on the reticle iswritten and overwritten. Typically, two to four passes are used to writea reticle to average out precision errors in the charged particle beamwriter, allowing the creation of more accurate photomasks. Alsotypically, the list of shots, including the dosages, is the same forevery pass. In one variation of multi-pass exposure, the lists of shotsmay vary among exposure passes, but the union of the shots in anyexposure pass covers the same area. Multi-pass writing can reduceover-heating of the resist coating the surface. Multi-pass writing alsoaverages out random errors of the charged particle beam writer.Multi-pass writing using different shot lists for different exposurepasses can also reduce the effects of certain systemic errors in thewriting process.

Current optical lithography writing machines typically reduce thephotomask pattern by a factor of four during the optical lithographicprocess. Therefore, patterns formed on a reticle or mask must be fourtimes larger than the size of the desired pattern on the substrate orwafer.

Current-technology charged particle beam writers, using conventionaltechniques, can resolve features as small as 100 nm. For featuressmaller than 100 nm, however, conventional writing techniques may failto accurately resolve features. Additionally, manufacturing variationmay produce unacceptable LER and critical dimension (CD) variation. Thiscan be a problem for both conventional optical lithography, where OPCmay produce SRAF's having mask dimensions smaller than 100 nm, and forEUV lithography, where the main mask patterns may be smaller than 100 nmand where mask specifications may be tighter than for masks used forconventional optical lithography.

SUMMARY OF THE DISCLOSURE

A method and system for fracturing or mask data preparation for chargedparticle beam lithography are disclosed in which a plurality of chargedparticle beam shots is determined that will form a pattern on a surfaceusing a multi-beam charged particle beam writer, where the sensitivityof the pattern on the surface to manufacturing variation is reduced byincreasing edge slope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a charged particle beam system;

FIG. 2 illustrates an example of an electro-optical schematic diagram ofa multi-beam exposure system;

FIG. 3A illustrates an example of a rectangular shot;

FIG. 3B illustrates an example of a circular character projection shot;

FIG. 3C illustrates an example of a trapezoidal shot;

FIG. 3D illustrates an example of a dragged shot;

FIG. 3E illustrates an example of a shot which is an array of circularpatterns;

FIG. 3F illustrates an example of a shot which is a sparse array ofrectangular patterns;

FIG. 4 illustrates an example of a multi-beam charged particle beamsystem;

FIG. 5A illustrates an example of a cross-sectional dosage graph,showing registered pattern widths for each of two resist thresholds;

FIG. 5B illustrates an example of a cross-sectional dosage graph similarto FIG. 5A, but with a higher dosage edge slope than in FIG. 5A;

FIG. 6A illustrates an example of a desired 100 nm line-end pattern tobe formed on a reticle;

FIG. 6B illustrates an example of a simulated pattern formed using shotsgenerated by fracturing the pattern of FIG. 6A using conventionaltechniques;

FIG. 7A illustrates an example of a desired 80 nm line-end pattern to beformed on a reticle;

FIG. 7B illustrates an example of a simulated pattern formed using shotsgenerated by fracturing the pattern of FIG. 7A using conventionaltechniques;

FIG. 8A illustrates an example of a desired 60 nm line-end pattern to beformed on a reticle;

FIG. 8B illustrates an example of a simulated pattern formed using shotsgenerated by fracturing the pattern of FIG. 8A using conventionaltechniques;

FIG. 9 illustrates various examples of groups of shots that may be usedto form a 80 nm line-end pattern;

FIG. 10 illustrates simulated patterns formed by the various shot groupsof FIG. 9;

FIG. 11A illustrates an example of a group of rectangular patterns to beformed on a surface;

FIG. 11B illustrates an example of how the patterns of FIG. 11A may beformed on a surface using conventional non-overlapping VSB shots, in thepresence of mid-range scattering;

FIG. 12A illustrates an example of a set of overlapping VSB shots thatmay be used to form the patterns of FIG. 11A on a surface;

FIG. 12B illustrates an example of a pattern that may be formed on asurface from the shots of FIG. 12A;

FIG. 13A illustrates an example of a contact or via pattern;

FIG. 13B illustrates dosages to expose the contact or via pattern ofFIG. 13A conventionally, for a first of two exposure passes using amulti-beam exposure system;

FIG. 13C illustrates dosages to expose the contact or via pattern ofFIG. 13A conventionally, for a second of two exposure passes using amulti-beam exposure system;

FIG. 13D illustrates the combined dosages from FIG. 13B pass 1 and FIG.13C pass 2 for conventional exposure of the contact or via pattern usinga multi-beam exposure system;

FIG. 13E illustrates example dosages to expose the contact or viapattern of FIG. 13A using an exemplary method, for a first of twoexposure passes using a multi-beam exposure system;

FIG. 13F illustrates example dosages to expose the contact or viapattern of FIG. 13A using an exemplary method, for a second of twoexposure passes using a multi-beam exposure system;

FIG. 13G illustrates the combined dosages from FIG. 13E pass 1 and FIG.13F pass 2 for exemplary exposure of the contact or via pattern using amulti-beam exposure system;

FIG. 13H illustrates example dosages to expose the contact or viapattern of FIG. 13A using an exemplary method, for a first of twoexposure passes using a multi-beam exposure system, using differentdosages than the example of FIG. 13E;

FIG. 13I illustrates example dosages to expose the contact or viapattern of FIG. 13A using an exemplary method, for a second of twoexposure passes using a multi-beam exposure system, using differentdosages than the example of FIG. 13F;

FIG. 13J illustrates the combined dosages from FIG. 13H pass 1 and FIG.13I pass 2 for exemplary exposure of the contact or via pattern using amulti-beam exposure system;

FIG. 14 illustrates a conceptual flow diagram of how to prepare asurface, such as a reticle, for use in fabricating a substrate such asan integrated circuit on a silicon wafer using optical lithography;

FIG. 15A illustrates a conceptual flow diagram of one method ofcombining model-based and conventional fracturing in the same design;and

FIG. 15B illustrates a conceptual flow diagram of another method ofcombining model-based and conventional fracturing in the same design.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes a method for enhancing the accuracy ofcharged particle beam exposure by use of overlapping shots. The presentinvention enhances the ability of charged particle beam systems toaccurately produce patterns smaller than 100 nm on a reticle, withacceptable CD variation and LER in light of manufacturing variation.Additionally, the present invention expands the process window ofmanufacturing variations under which these accurate patterns may beproduced.

Referring now to the drawings, wherein like numbers refer to like items,FIG. 1 illustrates an embodiment of a lithography system, such as acharged particle beam writer system, in this case an electron beamwriter system 10, that employs a variable shaped beam (VSB) tomanufacture a surface 12. The electron beam writer system 10 has anelectron beam source 14 that projects an electron beam 16 toward anaperture plate 18. The plate 18 has an aperture 20 formed therein whichallows the electron beam 16 to pass. Once the electron beam 16 passesthrough the aperture 20 it is directed or deflected by a system oflenses (not shown) as electron beam 22 toward another rectangularaperture plate or stencil mask 24. The stencil 24 has formed therein anumber of openings or apertures 26 that define various simple shapessuch as rectangles and triangles. Each aperture 26 formed in the stencil24 may be used to form a pattern in the surface 12 of a substrate 34,such as a silicon wafer, a reticle or other substrate. An electron beam30 emerges from one of the apertures 26 and passes through anelectromagnetic or electrostatic reduction lens 38, which reduces thesize of the pattern emerging from the aperture 26. In commonly availablecharged particle beam writer systems, the reduction factor is between 10and 60. The reduced electron beam 40 emerges from the reduction lens 38and is directed by a series of deflectors 42 onto the surface 12 as apattern 28. The surface 12 is coated with resist (not shown) whichreacts with the electron beam 40. The electron beam 22 may be directedto overlap a variable portion of an aperture 26, affecting the size andshape of the pattern 28. Blanking plates (not shown) are used to deflectthe beam 16 or the shaped beam 22 so to prevent the electron beam fromreaching the surface 12 during a period after each shot when the lensesdirecting the beam 22 and the deflectors 42 are being re-adjusted forthe succeeding shot. Typically the blanking plates are positioned so asto deflect the electron beam 16 to prevent it from illuminating aperture20. Conventionally, the blanking period may be a fixed length of time,or it may vary depending, for example, on how much the deflector 42 mustbe re-adjusted for the position of the succeeding shot.

In electron beam writer system 10, the substrate 34 is mounted on amovable platform or stage 32. The stage 32 allows substrate 34 to berepositioned so that patterns which are larger than the maximumdeflection capability or field size of the charged particle beam 40 maybe written to surface 12 in a series of subfields, where each subfieldis within the capability of deflector 42 to deflect the beam 40. In oneembodiment the substrate 34 may be a reticle. In this embodiment, thereticle, after being exposed with the pattern, undergoes variousmanufacturing steps through which it becomes a lithographic mask orphotomask. The mask may then be used in an optical lithography machineto project an image of the reticle pattern 28, generally reduced insize, onto a silicon wafer to produce an integrated circuit. Moregenerally, the mask is used in another device or machine to transfer thepattern 28 on to a substrate (not illustrated).

A charged particle beam system may expose a surface with a plurality ofindividually-controllable beams or beamlets. FIG. 2 illustrates anelectro-optical schematic diagram in which there are three chargedparticle beamlets 210. Associated with each beamlet 210 is a beamcontroller 220. Each beam controller 220 can, for example, allow itsassociated beamlet 210 to strike surface 230, and can also preventbeamlet 210 from striking the surface 230. In some embodiments, beamcontroller 220 may also control beam blur, magnification, size and/orshape of beamlet 210. In this disclosure, a charged particle beam systemwhich has a plurality of individually-controllable beamlets is called amulti-beam system. In some embodiments, charged particles from a singlesource may be sub-divided to form a plurality of beamlets 210. In otherembodiments a plurality of sources may be used to create the pluralityof beamlets 210. In some embodiments, beamlets 210 may be shaped by oneor more apertures, whereas in other embodiments there may be noapertures to shape the beamlets. Each beam controller 220 may allow theperiod of exposure of its associated beamlet to be controlledindividually. Generally the beamlets will be reduced in size by one ormore lenses (not shown) before striking the surface 230, which willtypically be coated with a resist. In some embodiments each beamlet mayhave a separate electro-optical lens, while in other embodiments aplurality of beamlets, including possibly all beamlets, will share anelectro-optical lens.

For purposes of this disclosure, a shot is the exposure of some surfacearea over a period of time. The area may be comprised of multiplediscontinuous smaller areas. A shot may be comprised of a plurality ofother shots which may or may not overlap, and which may or may not beexposed simultaneously. A shot may comprise a specified dose, or thedose may be unspecified. Shots may use a shaped beam, an unshaped beam,or a combination of shaped and unshaped beams. FIG. 3 illustrates somevarious types of shots. FIG. 3A illustrates an example of a rectangularshot 310. A VSB charged particle beam system can, for example, formrectangular shots in a variety of x and y dimensions. FIG. 3Billustrates an example of a character projection (CP) shot 320, which iscircular in this example. FIG. 3C illustrates an example of atrapezoidal shot 330. In one embodiment, shot 330 may be a created usinga raster-scanned charged particle beam, where the beam is scanned, forexample, in the x-direction as illustrated with scan lines 332. FIG. 3Dillustrates an example of a dragged shot 340, disclosed in U.S. PatentApplication Publication 2011-0089345. Shot 340 is formed by exposing thesurface with a curvilinear shaped beam 342 at an initial referenceposition 344, and then moving the shaped beam across the surface fromposition 344 to position 346. A dragged shot path may be, for example,linear, piecewise linear, or curvilinear.

FIG. 3E illustrates an example of a shot 350 that is an array ofcircular patterns 352. Shot 350 may be formed in a variety of ways,including multiple shots of a single circular CP character, one or moreshots of a CP character which is an array of circular apertures, and oneor more multi-beam shots using circular apertures. FIG. 3F illustratesan example of a shot 360 that is a sparse array of rectangular patterns362 and 364. Shot 360 may be formed in a variety of ways, including aplurality of VSB shots, a CP shot, and one or more multi-beam shotsusing rectangular apertures. In some embodiments of multi-beam, shot 360may comprise a plurality of interleaved groups of other multi-beamshots. For example, patterns 362 may be shot simultaneously, thenpatterns 364 may be shot simultaneously at a time different frompatterns 362.

FIG. 4 illustrates an embodiment of a charged particle beam exposuresystem 400. Charged particle beam system 400 is a multi-beam system, inwhich a plurality of individually-controllable shaped beams cansimultaneously expose a surface. Multi-beam system 400 has an electronbeam source 402 that creates an electron beam 404. The electron beam 404is directed toward aperture plate 408 by condenser 406, which mayinclude electrostatic and/or magnetic elements. Aperture plate 408 has aplurality of apertures 410 which are illuminated by electron beam 404,and through which electron beam 404 passes to form a plurality of shapedbeamlets 436. In some embodiments, aperture plate 408 may have hundredsor thousands of apertures 410. Although FIG. 4 illustrates an embodimentwith a single electron beam source 402, in other embodiments apertures410 may be illuminated by electrons from a plurality of electron beamsources. Apertures 410 may be rectangular, or may be of a differentshape, for example circular. The set of beamlets 436 then illuminates ablanking controller plate 432. The blanking controller plate 432 has aplurality of blanking controllers 434, each of which is aligned with abeamlet 436. Each blanking controller 434 can individually control itsassociated beamlet 436, so as to either allow the beamlet 436 to strikesurface 424, or to prevent the beamlet 436 from striking the surface424. The amount of time for which the beam strikes the surface controlsthe total energy or “dose” applied by that beamlet. Therefore, the doseof each beamlet may be independently controlled.

In FIG. 4 beamlets that are allowed to strike surface 424 areillustrated as beamlets 412. In one embodiment, the blanking controller434 prevents its beamlet 436 from striking the surface 424 by deflectingbeamlet 436 so that it is stopped by an aperture plate 416 whichcontains an aperture 418. In some embodiments, blanking plate 432 may bedirectly adjacent to aperture plate 408. In other embodiments, therelative locations of aperture plate 408 and blanking controller 432 maybe reversed from the position illustrated in FIG. 4, so that beam 404strikes the plurality of blanking controllers 434. A system of lensescomprising elements 414, 420, and 422 allows projection of the pluralityof beamlets 412 onto surface 424 of substrate 426, typically at areduced size compared to the plurality of apertures 410. Thereduced-size beamlets form a beamlet group 440 which strikes the surface424 to form a pattern that matches a pattern of a subset of apertures410, the subset being those apertures 410 for which correspondingblanking controllers 434 allow beamlets 436 to strike surface 424. InFIG. 4, beamlet group 440 has four beamlets illustrated for forming apattern on surface 424.

Substrate 426 is positioned on movable platform or stage 428, which canbe repositioned using actuators 430. By moving stage 428, beam 440 canexpose an area larger than the dimensions of the maximum size patternformed by beamlet group 440, using a plurality of exposures or shots. Insome embodiments, the stage 428 remains stationary during an exposure,and is then repositioned for a subsequent exposure. In otherembodiments, stage 428 moves continuously and at a variable velocity. Inyet other embodiments, stage 428 moves continuously but at a constantvelocity, which can increase the accuracy of the stage positioning. Forthose embodiments in which stage 428 moves continuously, a set ofdeflectors (not shown) may be used to move the beam to match thedirection and velocity of stage 428, allowing the beamlet group 440 toremain stationary with respect to surface 424 during an exposure. Instill other embodiments of multi-beam systems, individual beamlets in abeamlet group may be deflected across surface 424 independently fromother beamlets in the beamlet group.

Other types of multi-beam systems may create a plurality of unshapedbeamlets 436, such as by using a plurality of charged particle beamsources to create an array of Gaussian beamlets.

Referring again for FIG. 1, the minimum size pattern that can beprojected with reasonable accuracy onto a surface 12 is limited by avariety of short-range physical effects associated with the electronbeam writer system 10 and with the surface 12, which normally comprisesa resist coating on the substrate 34. These effects include forwardscattering, Coulomb effect, and resist diffusion. Beam blur, also calledβ_(f), is a term used to include all of these short-range effects. Themost modern electron beam writer systems can achieve an effective beamblur radius or β_(f) in the range of 20 nm to 30 nm. Forward scatteringmay constitute one quarter to one half of the total beam blur. Modernelectron beam writer systems contain numerous mechanisms to reduce eachof the constituent pieces of beam blur to a minimum. Since somecomponents of beam blur are a function of the calibration level of aparticle beam writer, the β_(f) of two particle beam writers of the samedesign may differ. The diffusion characteristics of resists may alsovary. Variation of β_(f) based on shot size or shot dose can besimulated and systemically accounted for. But there are other effectsthat cannot or are not accounted for, and they appear as randomvariation.

The shot dosage of a charged particle beam writer such as an electronbeam writer system is a function of the intensity of the beam source 14and the exposure time for each shot. Typically the beam intensityremains fixed, and the exposure time is varied to obtain variable shotdosages. Different areas in a shot may have different exposure times,such as in a multi-beam shot. The exposure time may be varied tocompensate for various long-range effects such as backscatter, fogging,and loading effects in a process called proximity effect correction(PEC). Electron beam writer systems usually allow setting an overalldosage, called a base dosage, which affects all shots in an exposurepass. Some electron beam writer systems perform dosage compensationcalculations within the electron beam writer system itself, and do notallow the dosage of each shot to be assigned individually as part of theinput shot list, the input shots therefore having unassigned shotdosages. In such electron beam writer systems all shots have the basedosage, before PEC. Other electron beam writer systems do allow dosageassignment on a shot-by-shot basis. In electron beam writer systems thatallow shot-by-shot dosage assignment, the number of available dosagelevels may be 64 to 4096 or more, or there may be a relatively fewavailable dosage levels, such as 3 to 8 levels.

The mechanisms within electron beam writers have a relatively coarseresolution for calculations. As such, mid-range corrections such as maybe required for EUV masks in the range of 2 μm cannot be computedaccurately by current electron beam writers.

Conventionally, shots are designed so as to completely cover an inputpattern with rectangular shots, while avoiding shot overlap whereverpossible. Also, all shots are designed to have a normal dosage, which isa dosage at which a relatively large rectangular shot, in the absence oflong-range effects, will produce a pattern on the surface which is thesame size as is the shot size.

In exposing, for example, a repeated pattern on a surface using chargedparticle beam lithography, the size of each pattern instance, asmeasured on the final manufactured surface, will be slightly different,due to manufacturing variations. The amount of the size variation is anessential manufacturing optimization criterion. In current mask masking,a root mean square (RMS) variation of no more than 1 nm (1 sigma) inpattern size may be desired. More size variation translates to morevariation in circuit performance, leading to higher design margins beingrequired, making it increasingly difficult to design faster, lower-powerintegrated circuits. This variation is referred to as critical dimension(CD) variation. A low CD variation is desirable, and indicates thatmanufacturing variations will produce relatively small size variationson the final manufactured surface. In the smaller scale, the effects ofa high CD variation may be observed as line edge roughness (LER). LER iscaused by each part of a line edge being slightly differentlymanufactured, leading to some waviness in a line that is intended tohave a straight edge. CD variation is, among other things, inverselyrelated to the slope of the dosage curve at the resist threshold, whichis called edge slope. Therefore, edge slope, or dose margin, is acritical optimization factor for particle beam writing of surfaces. Inthis disclosure, edge slope and dose margin are terms that are usedinterchangeably.

With conventional fracturing, without shot overlap, gaps or dosemodulation, the dose margin of the written shapes is consideredimmutable: that is, there is no opportunity to improve dose margin by achoice of fracturing options. In modern practice, the avoidance of verynarrow shots called slivers is an example of a practical rule-basedmethod that helps to optimize the shot list for dose margin.

In a fracturing environment where overlapping shots and dose-modulatedshots can be generated, there is both a need and an opportunity tooptimize for dose margin. The additional flexibility in shotcombinations allowed by use of shot overlap and dose modulation allowsgeneration of fracturing solutions that appear to generate the targetmask shapes on the surface, but may do so only under perfectmanufacturing conditions. The use of overlapping shots anddose-modulated shots therefore creates incentive to address the issue ofdose margin and its improvement.

FIGS. 5A-B illustrate how critical dimension variation can be reduced byexposing the pattern on the resist so as to produce a relatively highedge slope in the exposure or dosage curve. FIG. 5A illustrates across-sectional dosage curve 502, where the x-axis shows thecross-sectional distance through an exposed pattern—such as the distanceperpendicular to two of the pattern's edges—and the y-axis shows thedosage received by the resist. A pattern is registered by the resistwhere the received dosage is higher than a threshold. Two thresholds areillustrated in FIG. 5A, illustrating the effect of a variation in resistsensitivity. The higher threshold 504 causes a pattern of width 514 tobe registered by the resist. The lower threshold 506 causes a pattern ofwidth 516 to be registered by the resist, where width 516 is greaterthan width 514. Also illustrated in FIG. 5A is line segment 518 which istangent to dosage curve 502 at the intersection of dosage curve 502 andresist threshold 506. The slope m₁ of line segment 518 is Δy₁/Δx, whichis also the edge slope of dosage curve 502 at resist threshold 506. FIG.5B illustrates another cross-sectional dosage curve 522. Two thresholdsare illustrated, where threshold 524 is the same as threshold 504 ofFIG. 5A, and threshold 526 is the same as threshold 506 of FIG. 5A. Alsoillustrated in FIG. 5B is line segment 538 which is tangent to dosagecurve 522 at the intersection of dosage curve 522 and dosage 526. Theslope m₂ of line segment 538 is Δy₂/Δx, which is also the edge slope ofdosage curve 522 at resist threshold 526. As can be seen, the edge slopem₂ of dosage curve 522 at threshold 526 is greater than the edge slopem₁ of dosage curve 502 at threshold 506. For dosage curve 522, thehigher threshold 524 causes a pattern of width 534 to be registered bythe resist. The lower threshold 526 causes a pattern of width 536 to beregistered by the resist. As can be seen, the difference between width536 and width 534 is less than the difference between width 516 andwidth 514, due to the higher edge slope of dosage curve 522 compared todosage curve 502. If the resist-coated surface is a reticle, then thelower sensitivity of curve 522 to variation in resist threshold cancause the pattern width on a photomask manufactured from the reticle tobe closer to the target pattern width for the photomask, therebyincreasing the yield of usable integrated circuits when the photomask isused to transfer a pattern to a substrate such as a silicon wafer.Similar improvement in tolerance to variation in dose for each shot isobserved for dose curves with higher edge slopes. Achieving a relativelyhigher edge slope such as edge slope m₂ of dosage curve 522 at threshold526 is therefore desirable.

FIG. 6A illustrates an example of a designed pattern 602. Pattern 602 isdesigned to have a constant width 606, the width being 100 nm. Pattern602 comprises a line-end 604. FIG. 6B illustrates an example of asimulated pattern 612 that may be formed on a surface using aconventional VSB shot, where the VSB shot is a 100 nm wide rectangle,and of a normal dosage. As can be seen in FIG. 6B, the line-end portion614 of pattern 612 has rounded corners, due to beam blur caused by thephysical limitations of the charged particle beam writer. Additionally,the exposed pattern has a poor edge slope in sections 616 and 618 of thepattern perimeter. This edge slope may be determined, for example, usingparticle beam simulation. The portions 616 and 618 of the pattern 612may cause an undesirably-large variation in size due to manufacturingvariation. The line-end 614, in its center section, however, is thedesired length—i.e. having the same y-coordinate as the designedline-end 604.

FIG. 7A illustrates an example of a designed pattern 702. Pattern 702 isdesigned to have a constant width 706 of 80 nm. Pattern 702 comprises aline-end 704, where the y-coordinate of the line-end 704 is shown byreference line 708. FIG. 7B illustrates an example of a simulatedpattern 712 that may be formed on a surface using a conventional VSBshot, where the VSB shot is 80 nm wide, and of a normal dosage. As withpattern 612, the line-end portion 714 of pattern 712 has rounded cornersdue to beam blur. Also, the portions 716 and 718 of the perimeter ofpattern 712 have poor edge slope. As can be seen, the portions 716 and718 of the perimeter of pattern 712 having poor edge slope are largerthan the portions 616 and 618 of pattern 612 which have poor edge slope.This is due to the narrower 80 nm width of pattern 702 compared to the100 nm width of pattern 602. Additionally, the y-coordinate of formedline-end 714 is larger than the y-coordinate of the reference line 708,meaning that pattern 712 has line-end shortening, which can affect theperformance and/or functionality of an integrated circuit fabricatedusing a mask containing pattern 712.

FIG. 8A illustrates an example of a designed pattern 802. Pattern 802 isdesigned to have a constant width 808 of 60 nm. Pattern 802 comprises aline-end 804, where the y-coordinate of line-end 804 is shown byreference line 806. FIG. 8B illustrates an example of a pattern 812 thatmay be formed on a surface using a conventional VSB shot, where the VSBshot is 60 nm wide, and of a normal dosage. As can be seen, the line-endportion 814 of pattern 812 is very rounded. There is also line-endshortening—the minimum y-coordinate of pattern 812 is greater than they-coordinate of reference line 806. Additionally, the perimeter region818 of pattern 812 has a poor edge slope, affecting the entire line-end814.

The patterns of FIGS. 6B, 7B and 8B illustrate how formation of patternsof 80 nm width and below may have line-end shortening, and may also haverounded corners with poor edge slope, when formed with conventional VSBshots.

FIG. 9 illustrates various methods of fracturing a pattern to enhancethe quality of the pattern formed on a surface such as a reticle. Shape902 illustrates a designed line-end pattern, the pattern 902 having awidth 904 of 80 nm. The pattern 902 comprises a line-end 906. Dashedline 908 denotes the y-coordinate of line-end 906. FIG. 9 pattern 912illustrates one prior art method of fracturing pattern 902 to improvethe quality of the formed pattern on a surface, compared with FIG. 7Bpattern 712. Pattern 912 illustrates a single VSB shot, where the shotsize has been expanded in the negative y-dimension, so that the minimumy-coordinate of the shot is 7 nm less than reference y-coordinate 908.The dose of shot 912 is a normal dose. FIG. 10 pattern 1012 illustratesa simulated shape of the shot 912. The line end of pattern 1012 hasrounded corners, and also has perimeter regions 1014 and 1016 in whichthe edge slope of the pattern is too low.

FIG. 9 also illustrates three groups of VSB shots, group 922, group 932and group 942, which can form the pattern 902. Shot group 932 and shotgroup 942 exemplify one embodiment of the current invention while shotgroup 922 represents a prior art method. Shot group 922 consists of shot924 and shot 926, which do not overlap each other. Shot 924 is shot at1.2 times a normal dose, before long-range PEC, and shot 926 is shot ata normal dose. The width 928 of shot 924 is less than 904, and iscalculated so as to produce a pattern of width 904 on the surface withthe larger-than-normal dosage. Shot 926, as can be seen, is extended inthe negative and positive x-directions beyond the dimensions of shot 924and also beyond the dimensions of pattern 902. FIG. 10 pattern 1022illustrates the simulated pattern produced by shot group 922. Theline-end corners 1024 of pattern 1022 have a higher edge slope thanpattern 1012, with no part of the corner having a too-small edge slope.Additionally, though not illustrated, the higher-than-normal dose ofshot 924 improves the edge slope on the left and right sides of pattern1022 compared to pattern 1012. One method of determining the shots ofshot group 922 is through model-based fracturing, which is the use ofsimulation, such as charged particle beam simulation, to determine a setof shots which can form a desired pattern on a resist-coated surface, bydetermining through simulation the pattern which will be produced on thesurface from a given set of one or more shots, where some or all of theshots may have non-normal dosages. Alternatively, the shots of shotgroup 922 may be determined through rule-based methods. Model-basedfracturing, although relatively more compute-intensive than rule-basedfracturing, may determine a shot list that will produce a more accuratepattern on the surface, compared to a shot list determined usingrule-based methods.

FIG. 9 shot group 932 illustrates an exemplary method of fracturingpattern 902 according to one embodiment of the current invention. Shotgroup 932 consists of shot 934, shot 936 and shot 938. Shots 936 and 938are illustrated with shading for improved clarity. Shot 934 is shot at ahigher-than-normal dose, for example 1.2 times normal dose, and thewidth of shot 934 is calculated so as to produce a pattern of width 904on a surface. Shot 936 and shot 938 both overlap shot 934, and bothextend below reference y-coordinate 908. The overlap between, forexample, shot 934 and 936 is a partial overlap, meaning that the area ofintersection between shot 934 and shot 936 is different than eithershot. Shot 936 and shot 938 are shot at a normal dose in this example.FIG. 10 pattern 1032 illustrates a simulated pattern from shot group932. Compared to pattern 1022, pattern 1032 exhibits less cornerrounding, but also has worse edge slope on the corners, with the edgeslope being less than the minimum acceptable value in perimeter regions1034 and 1036. Shot group 932 illustrates how use of overlapping shotsand other-than-normal dosages may allow patterns to be formed withhigher-fidelity than using conventional non-overlapping shots withnormal dosages.

FIG. 9 shot group 942 illustrates another example for fracturing pattern902 according to the current invention, using partially overlappingshots. Shot group 942 consists of shots 944, 946, 948 and 950. Shots946, 948 and 950 are illustrated with shading for improved clarity. Likeshots 924 and 934, shot 944 uses a higher-than-normal dose such as of1.2× normal. Shots 946, 948 and 950 use a normal dose in this example.Shot 950 overlaps shot 944. Shots 946 and 948 extend beyond referencey-coordinate 908. FIG. 10 pattern 1042 illustrates a simulated patternfrom shot group 942. The corners 1044 of the pattern 1042 line-end areless rounded than, for example, the corers of pattern 1022.Additionally, the edge slope in the corner region is higher-than-minimumat all locations. Like shot group 932, shot group 942 illustrates howuse of overlapping shots combined with other-than-normal dosages mayallow patterns to be formed with higher-fidelity than with conventionalmethods or prior art methods such as illustrated with the method of shot912.

The solution described above and illustrated in FIG. 9 shot groups 932and 942 may be implemented even using a charged particle beam systemthat does not allow dosage assignment for individual shots. In oneembodiment of the present invention, a small number of dosages may beselected, for example two dosages such as 1.0× normal and 1.2× normal,and shots for each of these two dosages may be separated and exposed intwo separate exposure passes, where the base dosage for one exposurepass is 1.0× normal and the base dosage for the other exposure pass is1.2× normal. For example, in FIG. 9 shot group 932, shot 936 and shot938 may be assigned to a first exposure pass using a base dosage of 1.0×normal dosage, and shot 934 may be assigned to a second exposure passusing a base dosages of 1.2× normal dosage. In this embodiment, theunion of shots for any exposure pass will be different than the union ofshots for all of the exposure passes combined.

In other embodiments of the current invention, sensitivity to types ofmanufacturing variation other than dosage variation may be reduced byusing overlapping shots. Beam blur variation is an example of anothertype of manufacturing variation. Additionally, the methods of thecurrent invention may also be practiced using complex characterprojection (CP) shots, or with a combination of complex CP and VSBshots.

FIG. 11A illustrates an example of a group of rectangular patterns 1100to be formed on a surface. The group of patterns 1100 comprises sixcomplete rectangles, including rectangle 1102, rectangle 1104, rectangle1106, rectangle 1108, rectangle 1110 and rectangle 1112. Additionally,portions of four additional rectangles are illustrated: rectangle 1114,rectangle 1116, rectangle 1118 and rectangle 1120. As can be seen, therectangles are arranged in a regular pattern with columns, whereadjacent columns are separated by a space 1130, and where adjacentrectangles within a column are separated by a space 1132.

Pattern group 1100 can be written to a surface using conventionalnon-overlapping VSB shots, using one VSB shot for each pattern inpattern group 1100. FIG. 11A can therefore also be viewed as a group ofshots 1100, comprising shots 1102, 1104, 1106, 1108, 1110, 1112, 1114,1116, 1118, and 1120. FIG. 11B illustrates an example of a set ofsimulated patterns 1150 that may be produced from shot group 1100, inthe presence of mid-range scattering. Set of patterns 1150 comprises sixwhole patterns, including pattern 1152, pattern 1154, pattern 1156,pattern 1158, pattern 1160 and pattern 1162. Pattern group 1150 alsocomprises four additional patterns where only a portion of the patternis illustrated in FIG. 11B, including pattern 1164, pattern 1166,pattern 1168 and pattern 1170. The patterns in pattern group 1150exhibit corner rounding due to beam blur, one example of which is corner1172. Additionally, the middle portion, measured in the y-direction, ofeach pattern in the middle two columns is narrower in the x-directionthan is the rest of the pattern, as illustrated by middle portion 1174of pattern 1158. This narrowing is the result of less mid-rangescattering energy reaching the middle portion 1174 of pattern 1158 thanreaches other portions of pattern 1158. In pattern 1158, patternnarrowing in region 1174 is caused by the gap between shots 1114 and1106, and by the gap between shots 1118 and 1112. Less mid-rangescattering energy reaches the resist in the vicinity of pattern 1158opposite these gaps, compared to opposite shots 1114, 1106, 1118 and1112. Outside column patterns 1152, 1154, 1168, 1162 and 1170 exhibitasymmetrical narrowing because of their having neighboring shots on onlyone of the left or right sides. Inward-facing sides have a similarnarrowing as pattern 1158, as illustrated with narrowing region 1176 ofpattern 1162. On outside-facing edges such as edge 1178 of pattern 1162the lack of a neighboring pattern causes lower mid-range scatteringenergy to be received along the entire edge, with the consequence thatthe entire edge 1178 is offset in the −x (negative x) direction, causingthe width 1182 of pattern 1162 to be less than width 1180 of pattern1158. This simulated mid-range scattering is similar in range of effectto the mid-range scattering of reticles for EUV optical lithography, butthe mid-range scattering simulated in pattern group 1150 is of a higherintensity than current EUV reticles commonly produce. Pattern group 1150illustrates how mid-range scattering of a sufficient magnitude canaffect patterns written by charged particle beam lithography.

In another embodiment of the current invention, overlapping shots may beused to implement mask process correction, thereby producing higherfidelity patterns in the presence of mid-range scattering. FIG. 12Aillustrates a shot group 1200 that may be used to produce the group ofpatterns 1100. Shot group 1200 comprises rectangular shots 1202, 1204,1206, 1208, 1210 and 1212. Shot group 1200 also comprises rectangularshots 1214, 1216, 1218 and 1220, only portions of which are illustrated.Compared to shot group 1100, shot group 1200 includes the following:

-   -   Shots on the outside columns are widened on their outside edges.        This includes shots 1202, 1204, 1218, 1212 and 1220. In shot        1212, for example, edge 1236 has been moved in the +x direction,        compared to shot 1112.    -   Additional shots are added to prevent the pattern narrowing in        the middle portion of the patterns as illustrated in pattern        group 1150. The added shots include shots 1222, 1224, 1226,        1228, 1230 and 1232. These added shots deliver additional dosage        to areas, with the exception of outside edges of outside column        shots, that will receive less mid-range scattering dosage. Since        on the outside columns of shot group 1200, pattern narrowing is        prevented by widening shots 1202, 1204, 1218, 1212 and 1220 on        their outside edges as described above, overlapping shots 1222,        1224 and 1232 are positioned away from the outside edges of        shots 1202, 1204 and 1212 to prevent excessive middle-portion        widening of the patterns formed by shots 1202, 1204 and 1212.

FIG. 12B illustrates an example of a group of patterns 1250 that may beproduced on a surface from group of shots 1200. Group of patterns 1250comprises patterns 1252, 1254, 1256, 1258, 1260 and 1262, and partialpatterns 1264, 1266, 1268 and 1270. As can be seen, the exposure changesillustrated in group of shots 1200 compared to group of shots 1100improve the fidelity of the patterns produced on the surface, in thepresence of mid-range scattering. Narrowing of the middle portions ofpatterns is absent. Additionally, the widths of exterior columnpatterns, such as width 1282 of pattern 1262, are the same as the widthsof interior column patterns, such as width 1280 of pattern 1258.

FIG. 13A illustrates an example of a contact or via pattern 1302 that isto be exposed on a resist-coated surface using a multi-beam exposuresystem, using two exposure passes. In this example the multi-beamexposure system beamlets can expose pixels on a grid with 20 nm pixelspacing. The two exposure passes are offset 10 nm in both x and y,thereby producing an effective exposure grid of 10 nm. The pattern 1302is superimposed on a 10 nm pixel grid as shown by 1308. In this example,1.0 is a normal dosage, and the resist threshold is 0.5 times a normaldose. FIGS. 13B & 13C illustrate conventional exposure of pattern 1302.FIG. 13B illustrates conventional exposure for a first exposure pass1310 of two exposure passes, shown on grid 1312. As can be seen, adosage of 0.5 times a normal dose is used for all multi-beam beamlets orgrids which are within the perimeter of pattern 1302. The perimeteredges of pattern 1302 closely align with the grid squares of exposuregrid 1312. Exposure grid 1312 has a grid size 1314 of 20 nm. FIG. 13Cillustrates conventional exposure for the second exposure pass 1320,using a pixel grid 1322 in which the grid size 1358 is also 20 nm. Notethat the pixel alignment in grid 1322 is offset ½ pixel—10 nm—frompixels in pass 1 exposure grid 1312, in both x and y coordinates. Thisoffset is illustrated by pixel 1352, illustrated in dashed lines, whichhas pass 1 pixel alignment. The x-offset 1354 and the y-offset 1356 areboth 10 nm. The perimeter of pattern 1302 does not align with theboundaries of the grid squares of grid 1322. As can be seen the dosagesof pixels which are fully enclosed by pattern 1302 have a 0.5 dosage.Pixels or grid squares which are partially enclosed by pattern 1302 areassigned dosages in proportion to the fraction of each pixel which isenclosed by pattern 1302. FIG. 13D illustrates a calculated combinedexposure 1330 for each 10 nm grid based on the exposure from bothexposure passes, which in this example is calculated by adding the firstpass and second pass dosages. As can be seen, the highest dosage for apixel is 1.0. The combined exposure 1330 does not display simulateddosage, since no forward scattering effects, such as beam blur, aretaken into account.

FIGS. 13E and 13F illustrate an example of exemplary two pass exposure,in which edge slope is increased by varying the dosage of a firstbeamlet in the plurality of beamlets compared to the dosage of a secondbeamlet in the plurality of beamlets. FIG. 13E illustrates an example ofpixel dosages for pass 1 of 2 exposure passes, using a pixel grid 1340in which the grid size 1342 is 20 nm. FIG. 13F illustrates an example ofpixel dosages for exposure pass 2, using a pixel grid 1350. The size1358 of pixel grid 1350 is 20 nm. Like in FIG. 13C, grid squares in grid1350 are offset ½ pixel—10 nm—from pixels in pass 1 exposure grid 1340,in both x and y coordinates. This offset is illustrated by pixel 1352,illustrated in dashed lines, which has pass 1 pixel alignment. Thex-offset 1354 and the y-offset 1356 are both 10 nm. FIG. 13G illustratesthe combined exposure 1360 of passes 1 and 2, shown on a ½-pixel 10 nmgrid. As in FIG. 13D, pass 1 and pass 2 dosages are combined by addingthe dosages in each ½-pixel, and not are intended to show dosagereceived on the surface. Compared to conventional combined dosages 1330,combined dosages 1360 illustrate the following:

-   -   The maximum dosage of pixels near the perimeter of pattern 1302        in FIG. 13G is between 1.40 and 1.80, which is higher than the        maximum conventional combined dosage of 1.0 of FIG. 13D.    -   A higher dosage is used in corners. Simulation indicates that        this reduces corner rounding.    -   Pixels which are 20 nm or more toward the interior of pattern        1302 from the highest-dosage pixels have dosage <1.0 in FIG.        13G. This reduces back-scatter contribution. At the very center        of the figure, a 1×1 pixel (20 nm×20 nm) area receives only 0.28        times a normal dosage. When beam blur is taken into account,        simulation shows that no hole is registered by the resist, even        for a manufacturing variation in which the resist threshold was        0.7 times a normal dose, rather than 0.5.        The beamlet dosages for passes 1 and 2 may be determined using        model-based fracturing techniques.

Various solutions are possible which provide elevated dosage near theperimeters of patterns. FIGS. 13H and 13I illustrate another example ofexemplary two pass exposure. FIG. 13H illustrates an example of pixeldosages for pass 1 of 2 exposure passes, using a pixel grid 1370 inwhich the grid size 1342 is 20 nm. FIG. 13I illustrates an example ofpixel dosages for exposure pass 2, using a pixel grid 1380. The size1358 of pixel grid 1350 is 20 nm. Like in FIGS. 13C and 13F, gridsquares in grid 1380 are offset ½ pixel—10 nm—from pixels in pass 1exposure grid 1370, in both x and y coordinates. This offset isillustrated by pixel 1352, illustrated in dashed lines, which has pass 1pixel alignment. The x-offset 1354 and the y-offset 1356 are both 10 nm.FIG. 13J illustrates the combined exposure 1390 of passes 1 and 2, shownon a ½-pixel 10 nm grid. As in FIGS. 13D and 13G, pass 1 and pass 2dosages are combined by adding the dosages in each ½-pixel, and not areintended to show dosage received on the surface. Compared to FIG. 13G,the combined half-pixel dosages within 10 nm of the perimeter of pattern1302 are lower in FIG. 13J than in FIG. 13G. However, the combinedhalf-pixel dosages between 10 nm and 20 nm from the perimeter of pattern1302 are higher in FIG. 13J than in FIG. 13G, which may produce a higheredge slope in FIG. 13J than in FIG. 13G. Compared to the dosages of FIG.13G, the dosages of FIG. 13J may produce a pattern that less accuratelyfollows the perimeter of pattern 1302. However, the pattern produced bythe dosages of FIG. 13J may display less dimensional variation withmanufacturing variation than the pattern produced with dosages of FIG.13G.

The calculations described or referred to in this invention may beaccomplished in various ways. Generally, calculations may beaccomplished by in-process, pre-process or post-process methods.In-process calculation involves performing a calculation when itsresults are needed. Pre-process calculation involves pre-calculating andthen storing results for later retrieval during a subsequent processingstep, and may improve processing performance, particularly forcalculations that may be repeated many times. Calculations may also bedeferred from a processing step and then done in a later post-processingstep. An example of pre-process calculation is a shot group, which is apre-calculation of dosage pattern information for one or more shotsassociated with a given input pattern or set of input patterncharacteristics. The shot group and the associated input pattern may besaved in a library of pre-calculated shot groups, so that the set ofshots comprising the shot group can be quickly generated for additionalinstances of the input pattern, without pattern re-calculation. In someembodiments, the pre-calculation may comprise simulation of the dosagepattern that the shot group will produce on a resist-coated surface. Inother embodiments, the shot group may be determined without simulation,such as by using correct-by-construction techniques. In someembodiments, the pre-calculated shot groups may be stored in the shotgroup library in the form of a list of shots. In other embodiments, thepre-calculated shot groups may be stored in the form of computer codethat can generate shots for a specific type or types of input patterns.In yet other embodiments, a plurality of pre-calculated shot groups maybe stored in the form of a table, where entries in the table correspondto various input patterns or input pattern characteristics such aspattern width, and where each table entry provides either a list ofshots in the shot group, or information for how to generate theappropriate set of shots. Additionally, different shot groups may bestored in different forms in the shot group library. In someembodiments, the dosage pattern which a given shot group can produce mayalso be stored in the shot group library. In one embodiment, the dosagepattern may be stored as a two-dimensional (X and Y) dosage map called aglyph.

FIG. 14 is a conceptual flow diagram 1450 of how to prepare a reticlefor use in fabricating a surface such as an integrated circuit on asilicon wafer. In a first step 1452, a physical design, such as aphysical design of an integrated circuit, is designed. This can includedetermining the logic gates, transistors, metal layers, and other itemsthat are required to be found in a physical design such as that in anintegrated circuit. The physical design may be rectilinear, partiallycurvilinear, or completely curvilinear. Next, in a step 1454, opticalproximity correction is determined. In an embodiment of this disclosure,this can include taking as input a library of pre-calculated shot groupsfrom a shot group library 1474. This can also alternatively, or inaddition, include taking as input a library of pre-designed characters1480 including complex characters that are to be available on a stencil1484 in a step 1462. In an embodiment of this disclosure, an OPC step1454 may also include simultaneous optimization of shot count or writetimes, and may also include a fracturing operation, a shot placementoperation, a dose assignment operation, or may also include a shotsequence optimization operation, or other mask data preparationoperations, with some or all of these operations being simultaneous orcombined in a single step. The OPC step may create partially orcompletely curvilinear patterns. The output of the OPC step 1454 is amask design 1456.

Mask process correction (MPC) 1457 may optionally be performed on themask design 1456. MPC modifies the pattern to be written to the reticle,compensating for effects such as the narrowing of patterns which areless than about 100 nm wide. In a step 1458, a mask data preparation(MDP) operation which may include a fracturing operation, a shotplacement operation, a dose assignment operation, or a shot sequenceoptimization may take place. MDP may use as input the mask design 1456or the results of MPC 1457. In some embodiments of the presentinvention, MPC may be performed as part of a fracturing or other MDPoperation. Other corrections may also be performed as part of fracturingor other MDP operation, the possible corrections including: forwardscattering, resist diffusion, Coulomb effect, etching, backwardscattering, fogging, loading, resist charging, and EUV mid-rangescattering. MDP may comprise determining a set of multi-beam shots,where each multi-beam shot comprises a plurality of beamlets. In someembodiments, the set of multi-beam shots my comprise shots for aplurality of exposure passes. In one embodiment, the plurality ofexposure passes comprises a first pass and a second pass, where themulti-beam shots in the first pass are offset from the multi-beam shotsin the second pass by a distance which is a fraction of the pixelspacing between adjacent beamlets. The result of MDP step 1458 is a shotlist 1460. Either the OPC step 1454 or of the MDP step 1458, or aseparate program 1472 can include pre-calculating one or more shotgroups that may be used for a given input pattern, and storing thisinformation in a shot group library 1474. Combining OPC and any or allof the various operations of mask data preparation in one step iscontemplated in this disclosure. Mask data preparation step 1458, whichmay include a fracturing operation, may also comprise a pattern matchingoperation to match pre-calculated shot groups to create a mask thatmatches closely to the mask design. Mask data preparation may alsocomprise reducing the sensitivity of the pattern written in step 1462 tomanufacturing variation, which in some embodiments may comprisesincreasing edge slope. Mask data preparation may also comprise inputtingpatterns to be formed on a surface with the patterns being slightlydifferent, selecting a set of characters to be used to form the numberof patterns, the set of characters fitting on a stencil mask, the set ofcharacters possibly including both complex and VSB characters, and theset of characters based on varying character dose or varying characterposition or varying the beam blur radius or applying partial exposure ofa character within the set of characters or dragging a character toreduce the shot count or total write time. A set of slightly differentpatterns on the surface may be designed to produce substantially thesame pattern on a substrate. Also, the set of characters may be selectedfrom a predetermined set of characters. In one embodiment of thisdisclosure, a set of characters available on a stencil in a step 1480that may be selected quickly during the mask writing step 1462 may beprepared for a specific mask design. In that embodiment, once the maskdata preparation step 1458 is completed, a stencil is prepared in a step1484. In another embodiment of this disclosure, a stencil is prepared inthe step 1484 prior to or simultaneous with the MDP step 1458 and may beindependent of the particular mask design. In this embodiment, thecharacters available in the step 1480 and the stencil layout aredesigned in step 1482 to output generically for many potential maskdesigns 1456 to incorporate patterns that are likely to be output by aparticular OPC program 1454 or a particular MDP program 1458 orparticular types of designs that characterizes the physical design 1452such as memories, flash memories, system on chip designs, or particularprocess technology being designed to in physical design 1452, or aparticular cell library used in physical design 1452, or any othercommon characteristics that may form different sets of slightlydifferent patterns in mask design 1456. The stencil can include a set ofcharacters, such as a limited number of characters that was determinedin the step 1458.

The shot list 1460 is used to generate a surface in a mask writing step1462, which uses a charged particle beam writer such as an electron beamwriter system. Mask writing step 1462 may use stencil 1484 containing aplurality of complex characters, or may use a stencil comprising onlyVSB apertures, or may use a multi-beam system with either shapedbeamlets or unshaped beamlets. The electron beam writer system projectsa beam of electrons onto a surface to form patterns in a surface, asshown in a step 1464. One exposure pass or a plurality of exposurepasses may be used to form the patterns on the surface. The completedsurface may then be used in an optical lithography machine, which isshown in a step 1466. Finally, in a step 1468, a substrate such as asilicon wafer is produced. As has been previously described, in step1480 characters may be provided to the OPC step 1454 or the MDP step1458. The step 1480 also provides characters to a character and stencildesign step 1482 or a shot group generation step 1472. The character andstencil design step 1482 provides input to the stencil step 1484 and tothe characters step 1480. The shot group generation step 1472 providesinformation to the shot group library 1474. Also, a shot grouppre-calculation step 1472 may use as input the physical design 1452 orthe mask design 1456, and may pre-calculate one or more shot groups,which are stored in a shot group library 1474.

Model-based fracturing may be combined with conventional fracturing in asingle design. This allows, for example, model-based fracturing to beused in those areas where it can provide the greatest benefit, whileusing conventional fracturing, which is less computationally intensive,for other parts of the design. As previously indicated, in conventionalfracturing, shot overlap is avoided whenever possible, and all shotshave a normal dosage before long-range correction. In FIG. 15Aconceptual flow diagram 1500 illustrates one embodiment for howconventional and model-based fracturing may be combined. The input tothe combined fracturing process is mask design 1502. Mask design 1502may be mask design 1456 from FIG. 14, or it may be a part of mask design1456, or an altered form of mask design 1456 such as from MPC 1457.Conventional fracturing 1504 is performed on the mask design 1502 tocreate a conventional shot list 1506. Alternatively, conventionalfracturing may be performed on parts of the mask design 1502, leavingsome parts unfractured. A model-based fracturing step 1508 then inputsthe shot list 1506 and modifies, adds, or deletes shots in complex areasof a design. Complex areas may include, for example, areas with thesmallest patterns, or areas with curvilinear patterns. Complex areas mayalso include areas with high influence from mid-range scattering.Complex areas may also include “hot spots” of particular sensitivity inmanufacturing. The word “complex” in this context may not mean geometriccomplexity of the shapes. In some embodiments, the model-basedfracturing 1508 may include determining in which areas to modify and/orreplace conventional shots with model-base shots. In other embodimentsthe complex areas may be determined in a separate step 1512, eitherautomatically from mask design 1456, or manually. In any case,model-based fracturing 1508 generates shots, some of which partiallyoverlap other shots. The model-based fracturing may replace or modifysome or all of the conventional shots in the designated or determinedcomplex portions of the design with shots that have been determinedusing model-based techniques. The output of the model-based fracturingstep 1508 is a final shot list 1510, containing both conventional andmodel-based shots. Final shot list 1508 corresponds to FIG. 14 shot list1460. With regard to coarse grain parallel processing of the steps inconceptual flow diagram 1500, the mask design 1502 may be a partialdesign, or it may be the entire design where each of the steps may beperformed in parallel.

FIG. 15B conceptual flow diagram 1520 illustrates another embodiment ofhow conventional and model-based fracturing may be combined. The inputto the combined fracturing process is mask design 1522. Mask design 1522may be mask design 1456 from FIG. 14, or it may be a part of mask design1456, or an altered form of mask design 1456 such as from MPC 1457. InFIG. 15B the mask design 1522 is processed by pattern division step1524, which separates the pattern data into non-complex pattern area1526 and complex pattern area 1528. A conventional fracturing step 1530uses the non-complex pattern area 1526 as input. The conventionalfracturing 1530 outputs a list of conventional shots 1536. An additionaloutput is PEC information 1532. In some embodiments this information maybe one or more forms directly usable by PEC. In other embodiments, thePEC information may be, for example, the conventional shot list itself,from which PEC information may be calculated. The complex pattern area1528 is fractured using model-based fracturing 1534. Model-basedfracturing 1534 may use the PEC information 1532 as input, processingthis information if necessary to derive the appropriate PEC correctionsfor the model-based shots which are within the influence range of thelong-range effects from the conventional shots. In other embodiments,the PEC information may also be output by model-based fracturing 1534,and conventional fracturing 1530 may use this information in some way.Model-based fracturing 1534 creates a model-based shot list 1538. Theconventional shot list 1536 and the model-based shot list 1538 are thenmerged into a merged shot list 1540, which corresponds to FIG. 14 shotlist 1460. With regard to coarse grain parallel processing of the stepsin conceptual flow diagram 1520, the mask design 1522 may be a partialdesign, or it may be the entire design where each of the steps may beperformed in parallel.

The fracturing, mask data preparation, proximity effect correction andshot group creation flows described in this disclosure may beimplemented using general-purpose computers with appropriate computersoftware as computation devices. Due to the large amount of calculationsrequired, multiple computers or processor cores may also be used inparallel. In one embodiment, the computations may be subdivided into aplurality of 2-dimensional geometric regions for one or morecomputation-intensive steps in the flow, to support parallel processing.In another embodiment, a special-purpose hardware device, either usedsingly or in multiples, may be used to perform the computations of oneor more steps with greater speed than using general-purpose computers orprocessor cores. In one embodiment, the special-purpose hardware devicemay be a graphics processing unit (GPU). In another embodiment, theoptimization and simulation processes described in this disclosure mayinclude iterative processes of revising and recalculating possiblesolutions, so as to minimize either the total number of shots, or thetotal charged particle beam writing time, or some other parameter. Inyet another embodiment, an initial set of shots may be determined in acorrect-by-construction method, so that no shot modifications arerequired.

While the specification has been described in detail with respect tospecific embodiments, it will be appreciated that those skilled in theart, upon attaining an understanding of the foregoing, may readilyconceive of alterations to, variations of, and equivalents to theseembodiments. These and other modifications and variations to the presentmethods for fracturing, mask data preparation, proximity effectcorrection and optical proximity correction may be practiced by those ofordinary skill in the art, without departing from the scope of thepresent subject matter, which is more particularly set forth in theappended claims. Furthermore, those of ordinary skill in the art willappreciate that the foregoing description is by way of example only, andis not intended to be limiting. Steps can be added to, taken from ormodified from the steps in this specification without deviating from thescope of the invention. In general, any flowcharts presented are onlyintended to indicate one possible sequence of basic operations toachieve a function, and many variations are possible. Thus, it isintended that the present subject matter covers such modifications andvariations as come within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A method for fracturing or mask data preparationor mask process correction or proximity effect correction for chargedparticle beam lithography, the method comprising: determining aplurality of charged particle beam shots that will form a pattern on asurface using a multi-beam charged particle beam writer; wherein eachcharged particle beam shot is a multi-beam shot comprising a pluralityof beamlets; wherein a sensitivity of the pattern on the surface tomanufacturing variation is reduced by increasing edge slope; and whereinthe step of determining is performed using one or more computinghardware processors.
 2. The method of claim 1 wherein the determiningcomprises calculating the pattern on the surface from the plurality ofcharged particle beam shots.
 3. The method of claim 2 wherein thecalculating comprises charged particle beam simulation.
 4. The method ofclaim 3 wherein the charged particle beam simulation includes at leastone of a group of short-range effects consisting of forward scattering,resist diffusion, Coulomb effect, and etching.
 5. The method of claim 3wherein the surface is an extreme ultraviolet (EUV) reticle, and whereinthe charged particle beam simulation includes EUV mid-range scattering.6. The method of claim 1 wherein the edge slope is increased by varyingthe dosage of a first beamlet in the plurality of beamlets compared tothe dosage of a second beamlet in the plurality of beamlets.
 7. Themethod of claim 1 wherein the plurality of charged particle beam shotscomprises a plurality of multi-beam shots in each of a plurality ofexposure passes.
 8. The method of claim 7 wherein a first distancebetween adjacent beamlets in the plurality of beamlets comprises a pixelspacing, wherein the plurality of exposure passes comprises a first passand a second pass, and wherein multi-beam shots in the first pass areoffset from multi-beam shots in the second pass by a second distancewhich is a fractional pixel spacing.
 9. The method of claim 8 whereinthe second distance is one-half of a pixel spacing.
 10. A method formanufacturing a surface using charged particle beam lithography, themethod comprising: determining a plurality of charged particle beamshots that will form a pattern on a surface using a multi-beam chargedparticle beam writer; and forming the pattern on the surface with theplurality of charged particle beam shots; wherein each charged particlebeam shot is a multi-beam shot comprising a plurality of beamlets;wherein the sensitivity of the pattern on the surface to manufacturingvariation is reduced by increasing edge slope; and wherein the step ofdetermining is performed using one or more computing hardwareprocessors.
 11. The method of claim 10 wherein the determining comprisescalculating the pattern on the surface from the plurality of chargedparticle beam shots.
 12. The method of claim 11 wherein the calculationcomprises charged particle beam simulation.
 13. The method of claim 12wherein the charged particle beam simulation includes at least one of agroup of short-range effects consisting of forward scattering, resistdiffusion, Coulomb effect, and etching.
 14. The method of claim 12wherein the surface is an extreme ultraviolet (EUV) reticle, and whereinthe charged particle beam simulation includes EUV mid-range scattering.15. The method of claim 10 wherein the edge slope is increased byvarying the dosage of a first beamlet in the plurality of beamletscompared to the dosage of a second beamlet in the plurality of beamlets.16. The method of claim 10 wherein the plurality of charged particlebeam shots comprises a plurality of multi-beam shots in each of aplurality of exposure passes.
 17. A method for manufacturing anintegrated circuit using an optical lithographic process, the opticallithographic process using a reticle manufactured with charged particlebeam lithography, the method comprising: determining a plurality ofcharged particle beam shots that will form a pattern on the reticleusing a multi-beam charged particle beam writer; and forming the patternon the reticle with the plurality of charged particle beam shots;wherein each charged particle beam shot is a multi-beam shot comprisinga plurality of beamlets; wherein the sensitivity of the pattern on thereticle to manufacturing variation is reduced by increasing edge slope;and wherein the step of determining is performed using one or morecomputing hardware processors.
 18. The method of claim 17 wherein thedetermining comprises calculating the pattern on the reticle from theplurality of charged particle beam shots.
 19. The method of claim 18wherein the calculation comprises charged particle beam simulation. 20.The method of claim 19 wherein the charged particle beam simulationincludes at least one of a group of short-range effects consisting offorward scattering, resist diffusion, Coulomb effect, and etching. 21.The method of claim 19 wherein the reticle is an extreme ultraviolet(EUV) reticle, and wherein the charged particle beam simulation includesEUV mid-range scattering.
 22. The method of claim 17 wherein the edgeslope is increased by varying the dosage of a first beamlet in theplurality of beamlets compared to the dosage of a second beamlet in theplurality of beamlets.
 23. The method of claim 17 wherein the pluralityof charged particle beam shots comprises a plurality of multi-beam shotsin each of a plurality of exposure passes.
 24. A system for fracturingor mask data preparation or mask process correction or proximity effectcorrection for charged particle beam lithography comprising: a deviceconfigured to determine a plurality of charged particle beam shots thatwill form a pattern on a surface using a multi-beam charged particlebeam writer; wherein each charged particle beam shot is a multi-beamshot comprising a plurality of beamlets; and wherein the sensitivity ofthe pattern on the surface to manufacturing variation is reduced byincreasing edge slope.
 25. The system of claim 24 further comprising adevice configured to calculate the pattern on the surface from theplurality of charged particle beam shots.
 26. The system of claim 25wherein the device configured to calculate performs charged particlebeam simulation.